Provide flatMapIndexed operation

- similar to flatMap, but transform function takes index and element

#KT-36894

libraries/stdlib/api/js-v1/kotlin.collections.kt

@@ -2644,6 +2644,198 @@ public inline fun <K, V, R> kotlin.collections.Map<out K, V>.flatMap(transform:
 @kotlin.jvm.JvmName(name = "flatMapSequence")
 public inline fun <K, V, R> kotlin.collections.Map<out K, V>.flatMap(transform: (kotlin.collections.Map.Entry<K, V>) -> kotlin.sequences.Sequence<R>): kotlin.collections.List<R>
 
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.Array<out T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequence")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.Array<out T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.BooleanArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Boolean) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.ByteArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Byte) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.CharArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.DoubleArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Double) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.FloatArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Float) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.IntArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Int) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.LongArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Long) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.ShortArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Short) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.UByteArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.UByte) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.UIntArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.UInt) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.ULongArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.ULong) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.UShortArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.UShort) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.collections.Iterable<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequence")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.collections.Iterable<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.Array<out T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.Array<out T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.BooleanArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Boolean) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.ByteArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Byte) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.CharArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.DoubleArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Double) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.FloatArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Float) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.IntArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Int) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.LongArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Long) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.ShortArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Short) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.UByteArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.UByte) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.UIntArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.UInt) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.ULongArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.ULong) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.UShortArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.UShort) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.collections.Iterable<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.collections.Iterable<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): C
+
 public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.Array<out T>.flatMapTo(destination: C, transform: (T) -> kotlin.collections.Iterable<R>): C
 
 @kotlin.SinceKotlin(version = "1.4")

libraries/stdlib/api/js-v1/kotlin.sequences.kt

@@ -151,6 +151,28 @@ public fun <T, R> kotlin.sequences.Sequence<T>.flatMap(transform: (T) -> kotlin.
 
 public fun <T, R> kotlin.sequences.Sequence<T>.flatMap(transform: (T) -> kotlin.sequences.Sequence<R>): kotlin.sequences.Sequence<R>
 
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+public fun <T, R> kotlin.sequences.Sequence<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): kotlin.sequences.Sequence<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequence")
+public fun <T, R> kotlin.sequences.Sequence<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): kotlin.sequences.Sequence<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.sequences.Sequence<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.sequences.Sequence<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): C
+
 @kotlin.SinceKotlin(version = "1.4")
 @kotlin.OverloadResolutionByLambdaReturnType
 @kotlin.jvm.JvmName(name = "flatMapIterableTo")

libraries/stdlib/api/js-v1/kotlin.text.kt

@@ -263,6 +263,18 @@ public inline fun kotlin.CharSequence.firstOrNull(predicate: (kotlin.Char) -> ko
 
 public inline fun <R> kotlin.CharSequence.flatMap(transform: (kotlin.Char) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
 
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.CharSequence.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.CharSequence.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): C
+
 public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.CharSequence.flatMapTo(destination: C, transform: (kotlin.Char) -> kotlin.collections.Iterable<R>): C
 
 public inline fun <R> kotlin.CharSequence.fold(initial: R, operation: (acc: R, kotlin.Char) -> R): R

libraries/stdlib/api/js/kotlin.collections.kt

@@ -2614,6 +2614,198 @@ public inline fun <K, V, R> kotlin.collections.Map<out K, V>.flatMap(transform:
 @kotlin.jvm.JvmName(name = "flatMapSequence")
 public inline fun <K, V, R> kotlin.collections.Map<out K, V>.flatMap(transform: (kotlin.collections.Map.Entry<K, V>) -> kotlin.sequences.Sequence<R>): kotlin.collections.List<R>
 
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.Array<out T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequence")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.Array<out T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.BooleanArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Boolean) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.ByteArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Byte) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.CharArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.DoubleArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Double) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.FloatArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Float) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.IntArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Int) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.LongArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Long) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.ShortArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Short) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.UByteArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.UByte) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.UIntArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.UInt) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.ULongArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.ULong) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.UShortArray.flatMapIndexed(transform: (index: kotlin.Int, kotlin.UShort) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.collections.Iterable<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequence")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> kotlin.collections.Iterable<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.Array<out T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.Array<out T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.BooleanArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Boolean) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.ByteArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Byte) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.CharArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.DoubleArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Double) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.FloatArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Float) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.IntArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Int) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.LongArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Long) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.ShortArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Short) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.UByteArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.UByte) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.UIntArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.UInt) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.ULongArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.ULong) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.UShortArray.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.UShort) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.collections.Iterable<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.collections.Iterable<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): C
+
 public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.Array<out T>.flatMapTo(destination: C, transform: (T) -> kotlin.collections.Iterable<R>): C
 
 @kotlin.SinceKotlin(version = "1.4")

libraries/stdlib/api/js/kotlin.sequences.kt

@@ -151,6 +151,28 @@ public fun <T, R> kotlin.sequences.Sequence<T>.flatMap(transform: (T) -> kotlin.
 
 public fun <T, R> kotlin.sequences.Sequence<T>.flatMap(transform: (T) -> kotlin.sequences.Sequence<R>): kotlin.sequences.Sequence<R>
 
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+public fun <T, R> kotlin.sequences.Sequence<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): kotlin.sequences.Sequence<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequence")
+public fun <T, R> kotlin.sequences.Sequence<T>.flatMapIndexed(transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): kotlin.sequences.Sequence<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.sequences.Sequence<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.collections.Iterable<R>): C
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : kotlin.collections.MutableCollection<in R>> kotlin.sequences.Sequence<T>.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, T) -> kotlin.sequences.Sequence<R>): C
+
 @kotlin.SinceKotlin(version = "1.4")
 @kotlin.OverloadResolutionByLambdaReturnType
 @kotlin.jvm.JvmName(name = "flatMapIterableTo")

libraries/stdlib/api/js/kotlin.text.kt

@@ -263,6 +263,18 @@ public inline fun kotlin.CharSequence.firstOrNull(predicate: (kotlin.Char) -> ko
 
 public inline fun <R> kotlin.CharSequence.flatMap(transform: (kotlin.Char) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
 
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> kotlin.CharSequence.flatMapIndexed(transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): kotlin.collections.List<R>
+
+@kotlin.SinceKotlin(version = "1.4")
+@kotlin.OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName(name = "flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.CharSequence.flatMapIndexedTo(destination: C, transform: (index: kotlin.Int, kotlin.Char) -> kotlin.collections.Iterable<R>): C
+
 public inline fun <R, C : kotlin.collections.MutableCollection<in R>> kotlin.CharSequence.flatMapTo(destination: C, transform: (kotlin.Char) -> kotlin.collections.Iterable<R>): C
 
 public inline fun <R> kotlin.CharSequence.fold(initial: R, operation: (acc: R, kotlin.Char) -> R): R

libraries/stdlib/common/src/generated/_Arrays.kt

@@ -10035,6 +10035,336 @@ public inline fun <T, R> Array<out T>.flatMap(transform: (T) -> Sequence<R>): Li
     return flatMapTo(ArrayList<R>(), transform)
 }
 
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> Array<out T>.flatMapIndexed(transform: (index: Int, T) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> ByteArray.flatMapIndexed(transform: (index: Int, Byte) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> ShortArray.flatMapIndexed(transform: (index: Int, Short) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> IntArray.flatMapIndexed(transform: (index: Int, Int) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> LongArray.flatMapIndexed(transform: (index: Int, Long) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> FloatArray.flatMapIndexed(transform: (index: Int, Float) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> DoubleArray.flatMapIndexed(transform: (index: Int, Double) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> BooleanArray.flatMapIndexed(transform: (index: Int, Boolean) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> CharArray.flatMapIndexed(transform: (index: Int, Char) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedSequence")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> Array<out T>.flatMapIndexed(transform: (index: Int, T) -> Sequence<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : MutableCollection<in R>> Array<out T>.flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> ByteArray.flatMapIndexedTo(destination: C, transform: (index: Int, Byte) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> ShortArray.flatMapIndexedTo(destination: C, transform: (index: Int, Short) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> IntArray.flatMapIndexedTo(destination: C, transform: (index: Int, Int) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> LongArray.flatMapIndexedTo(destination: C, transform: (index: Int, Long) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> FloatArray.flatMapIndexedTo(destination: C, transform: (index: Int, Float) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> DoubleArray.flatMapIndexedTo(destination: C, transform: (index: Int, Double) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> BooleanArray.flatMapIndexedTo(destination: C, transform: (index: Int, Boolean) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> CharArray.flatMapIndexedTo(destination: C, transform: (index: Int, Char) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : MutableCollection<in R>> Array<out T>.flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Sequence<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
 /**
  * Appends all elements yielded from results of [transform] function being invoked on each element of original array, to the given [destination].
  */

libraries/stdlib/common/src/generated/_Collections.kt

@@ -1299,6 +1299,72 @@ public inline fun <T, R> Iterable<T>.flatMap(transform: (T) -> Sequence<R>): Lis
     return flatMapTo(ArrayList<R>(), transform)
 }
 
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original collection.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> Iterable<T>.flatMapIndexed(transform: (index: Int, T) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original collection.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedSequence")
+@kotlin.internal.InlineOnly
+public inline fun <T, R> Iterable<T>.flatMapIndexed(transform: (index: Int, T) -> Sequence<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original collection, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : MutableCollection<in R>> Iterable<T>.flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(checkIndexOverflow(index++), element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original collection, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : MutableCollection<in R>> Iterable<T>.flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Sequence<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(checkIndexOverflow(index++), element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
 /**
  * Appends all elements yielded from results of [transform] function being invoked on each element of original collection, to the given [destination].
  */

libraries/stdlib/common/src/generated/_Sequences.kt

@@ -777,7 +777,7 @@ public fun <T> Sequence<T>.toSet(): Set<T> {
 @OverloadResolutionByLambdaReturnType
 @kotlin.jvm.JvmName("flatMapIterable")
 public fun <T, R> Sequence<T>.flatMap(transform: (T) -> Iterable<R>): Sequence<R> {
-    return FlatteningSequence(this, transform, { it.iterator() })
+    return FlatteningSequence(this, transform, Iterable<R>::iterator)
 }
 
 /**
@@ -788,7 +788,79 @@ public fun <T, R> Sequence<T>.flatMap(transform: (T) -> Iterable<R>): Sequence<R
  * @sample samples.collections.Collections.Transformations.flatMap
  */
 public fun <T, R> Sequence<T>.flatMap(transform: (T) -> Sequence<R>): Sequence<R> {
-    return FlatteningSequence(this, transform, { it.iterator() })
+    return FlatteningSequence(this, transform, Sequence<R>::iterator)
+}
+
+/**
+ * Returns a single sequence of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original sequence.
+ *
+ * The operation is _intermediate_ and _stateless_.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+public fun <T, R> Sequence<T>.flatMapIndexed(transform: (index: Int, T) -> Iterable<R>): Sequence<R> {
+    return flatMapIndexed(this, transform, Iterable<R>::iterator)
+}
+
+/**
+ * Returns a single sequence of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original sequence.
+ *
+ * The operation is _intermediate_ and _stateless_.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedSequence")
+public fun <T, R> Sequence<T>.flatMapIndexed(transform: (index: Int, T) -> Sequence<R>): Sequence<R> {
+    return flatMapIndexed(this, transform, Sequence<R>::iterator)
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original sequence, to the given [destination].
+ *
+ * The operation is _terminal_.
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : MutableCollection<in R>> Sequence<T>.flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(checkIndexOverflow(index++), element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original sequence, to the given [destination].
+ *
+ * The operation is _terminal_.
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedSequenceTo")
+@kotlin.internal.InlineOnly
+public inline fun <T, R, C : MutableCollection<in R>> Sequence<T>.flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Sequence<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(checkIndexOverflow(index++), element)
+        destination.addAll(list)
+    }
+    return destination
 }
 
 /**

libraries/stdlib/common/src/generated/_Strings.kt

@@ -785,6 +785,39 @@ public inline fun <R> CharSequence.flatMap(transform: (Char) -> Iterable<R>): Li
     return flatMapTo(ArrayList<R>(), transform)
 }
 
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each character
+ * and its index in the original char sequence.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterable")
+@kotlin.internal.InlineOnly
+public inline fun <R> CharSequence.flatMapIndexed(transform: (index: Int, Char) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each character
+ * and its index in the original char sequence, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@kotlin.jvm.JvmName("flatMapIndexedIterableTo")
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> CharSequence.flatMapIndexedTo(destination: C, transform: (index: Int, Char) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
 /**
  * Appends all elements yielded from results of [transform] function being invoked on each character of original char sequence, to the given [destination].
  */

libraries/stdlib/common/src/generated/_UArrays.kt

@@ -4494,6 +4494,138 @@ public inline fun <R> UShortArray.flatMap(transform: (UShort) -> Iterable<R>): L
     return flatMapTo(ArrayList<R>(), transform)
 }
 
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> UIntArray.flatMapIndexed(transform: (index: Int, UInt) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> ULongArray.flatMapIndexed(transform: (index: Int, ULong) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> UByteArray.flatMapIndexed(transform: (index: Int, UByte) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Returns a single list of all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMapIndexed
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R> UShortArray.flatMapIndexed(transform: (index: Int, UShort) -> Iterable<R>): List<R> {
+    return flatMapIndexedTo(ArrayList<R>(), transform)
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> UIntArray.flatMapIndexedTo(destination: C, transform: (index: Int, UInt) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> ULongArray.flatMapIndexedTo(destination: C, transform: (index: Int, ULong) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> UByteArray.flatMapIndexedTo(destination: C, transform: (index: Int, UByte) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
+/**
+ * Appends all elements yielded from results of [transform] function being invoked on each element
+ * and its index in the original array, to the given [destination].
+ */
+@SinceKotlin("1.4")
+@OptIn(kotlin.experimental.ExperimentalTypeInference::class)
+@OverloadResolutionByLambdaReturnType
+@ExperimentalUnsignedTypes
+@kotlin.internal.InlineOnly
+public inline fun <R, C : MutableCollection<in R>> UShortArray.flatMapIndexedTo(destination: C, transform: (index: Int, UShort) -> Iterable<R>): C {
+    var index = 0
+    for (element in this) {
+        val list = transform(index++, element)
+        destination.addAll(list)
+    }
+    return destination
+}
+
 /**
  * Appends all elements yielded from results of [transform] function being invoked on each element of original array, to the given [destination].
  */

libraries/stdlib/samples/test/samples/collections/collections.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
 package samples.collections
@@ -549,6 +538,14 @@ class Collections {
             assertPrints(list.flatMap { it.toList() }, "[1, 2, 3, 4, 5]")
         }
 
+        @Sample
+        fun flatMapIndexed() {
+            val data: List<String> = listOf("Abcd", "efgh", "Klmn")
+            val selected: List<Boolean> = data.map { it.any { c -> c.isUpperCase() } }
+            val result = data.flatMapIndexed { index, s -> if (selected[index]) s.toList() else emptyList() }
+            assertPrints(result, "[A, b, c, d, K, l, m, n]")
+        }
+
         @Sample
         fun take() {
             val chars = ('a'..'z').toList()

libraries/stdlib/src/kotlin/collections/Sequences.kt

@@ -318,6 +318,15 @@ constructor(
     }
 }
 
+internal fun <T, C, R> flatMapIndexed(source: Sequence<T>, transform: (Int, T) -> C, iterator: (C) -> Iterator<R>): Sequence<R> =
+    sequence {
+        var index = 0
+        for (element in source) {
+            val result = transform(checkIndexOverflow(index++), element)
+            yieldAll(iterator(result))
+        }
+    }
+
 /**
  * A sequence that supports drop(n) and take(n) operations
  */

libraries/stdlib/test/collections/ArraysTest.kt

@@ -1786,6 +1786,15 @@ class ArraysTest {
         assertEquals(expected, result2)
     }
 
+    @Test fun flatMapIndexed() {
+        val data = arrayOf(arrayOf(1, 2, 3), arrayOf(4, 5, 6))
+        val result1 = data.flatMapIndexed { index, arr -> arr.asList().subList(0, index + 1) }
+        val result2 = data.flatMapIndexed { index, arr -> arr.asSequence().take(index + 1) }
+        val expected = listOf(1, 4, 5)
+        assertEquals(expected, result1)
+        assertEquals(expected, result2)
+    }
+
     @Test fun flattenArray() {
         val arr1: Array<Array<Int>> = arrayOf(arrayOf(1, 2, 3), arrayOf(4, 5, 6))
         val arr2: Array<out Array<Int>> = arr1

libraries/stdlib/test/collections/CollectionTest.kt

@@ -60,6 +60,16 @@ class CollectionTest {
         assertEquals(expected, result2)
     }
 
+    @Test fun flatMapIndexed() {
+        val source = listOf(null, "foo", "bar")
+        val result1 = source.flatMapIndexed { index, it -> it.orEmpty().take(index + 1).asSequence() }
+        val result2 = source.flatMapIndexed { index, it -> it.orEmpty().take(index + 1).asIterable() }
+
+        val expected = "fobar".toList()
+        assertEquals(expected, result1)
+        assertEquals(expected, result2)
+    }
+
     /*
     @Test fun mapNotNull() {
         val data = listOf(null, "foo", null, "bar")

libraries/stdlib/test/collections/SequenceTest.kt

@@ -592,6 +592,14 @@ public class SequenceTest {
         assertEquals(expected, result2.toList())
     }
 
+    @Test fun flatMapIndexed() {
+        val result1 = sequenceOf(1, 2).flatMapIndexed { index, v -> (0..v + index).asSequence() }
+        val result2 = sequenceOf(1, 2).flatMapIndexed { index, v -> 0..v + index }
+        val expected = listOf(0, 1, 0, 1, 2, 3)
+        assertEquals(expected, result1.toList())
+        assertEquals(expected, result2.toList())
+    }
+
     @Test fun flatten() {
         val expected = listOf(0, 1, 0, 1, 2)
 

libraries/stdlib/test/collections/UnsignedArraysTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -981,6 +981,14 @@ class UnsignedArraysTest {
         assertEquals(listOf(), ulongArrayOf(1, 2, 3).flatMap { listOf<ULong>() })
     }
 
+    @Test
+    fun flatMapIndexed() {
+        assertEquals(listOf(), ubyteArrayOf().flatMapIndexed { index, _ -> listOf(index) })
+        assertEquals(listOf<UShort>(2, 3, 3), ushortArrayOf(1, 2, 3).flatMapIndexed { index, e -> List(index) { e } })
+        assertEquals(listOf<UInt>(2, 2, 3, 3, 3, 3), uintArrayOf(1, 2, 3).flatMapIndexed { index, e -> List(index * 2) { e } })
+        assertEquals(listOf(), ulongArrayOf(1, 2, 3).flatMapIndexed { _, _ -> listOf<ULong>() })
+    }
+
     @Test
     fun withIndex() {
         fun <T> assertIterableContentEquals(expected: Iterable<T>, actual: Iterable<T>) {

libraries/stdlib/test/text/StringTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -1242,6 +1242,11 @@ class StringTest {
         assertEquals("a ab abc abcd ".toList(), result)
     }
 
+    @Test fun flatMapIndexed() = withOneCharSequenceArg("bdfi") { data ->
+        val result = data.flatMapIndexed { index, c -> ('a'..c).drop(index) + ' ' }
+        assertEquals("ab bcd cdef defghi ".toList(), result)
+    }
+
     @Test fun fold() = withOneCharSequenceArg { arg1 ->
         // calculate number of digits in the string
         val data = arg1("a1b2c3def")

libraries/tools/binary-compatibility-validator/reference-public-api/kotlin-stdlib-runtime-merged.txt

@@ -4880,6 +4880,8 @@ public final class kotlin/sequences/SequencesKt {
 	public static final fun firstOrNull (Lkotlin/sequences/Sequence;)Ljava/lang/Object;
 	public static final fun firstOrNull (Lkotlin/sequences/Sequence;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;
 	public static final fun flatMap (Lkotlin/sequences/Sequence;Lkotlin/jvm/functions/Function1;)Lkotlin/sequences/Sequence;
+	public static final fun flatMapIndexedIterable (Lkotlin/sequences/Sequence;Lkotlin/jvm/functions/Function2;)Lkotlin/sequences/Sequence;
+	public static final fun flatMapIndexedSequence (Lkotlin/sequences/Sequence;Lkotlin/jvm/functions/Function2;)Lkotlin/sequences/Sequence;
 	public static final fun flatMapIterable (Lkotlin/sequences/Sequence;Lkotlin/jvm/functions/Function1;)Lkotlin/sequences/Sequence;
 	public static final fun flatMapIterableTo (Lkotlin/sequences/Sequence;Ljava/util/Collection;Lkotlin/jvm/functions/Function1;)Ljava/util/Collection;
 	public static final fun flatMapTo (Lkotlin/sequences/Sequence;Ljava/util/Collection;Lkotlin/jvm/functions/Function1;)Ljava/util/Collection;

libraries/tools/kotlin-stdlib-gen/src/templates/Mapping.kt

@@ -311,7 +311,7 @@ object Mapping : TemplateGroupBase() {
             doc { "Returns a single sequence of all elements from results of [transform] function being invoked on each element of original sequence." }
             returns("Sequence<R>")
             body {
-                "return FlatteningSequence(this, transform, { it.iterator() })"
+                "return FlatteningSequence(this, transform, Iterable<R>::iterator)"
             }
         }
     }
@@ -338,7 +338,7 @@ object Mapping : TemplateGroupBase() {
             doc { "Returns a single sequence of all elements from results of [transform] function being invoked on each element of original sequence." }
             returns("Sequence<R>")
             body {
-                "return FlatteningSequence(this, transform, { it.iterator() })"
+                "return FlatteningSequence(this, transform, Sequence<R>::iterator)"
             }
         }
     }
@@ -396,70 +396,79 @@ object Mapping : TemplateGroupBase() {
         }
     }
 
-    val f_flatMapIndexed = fn("flatMapIndexed(transform: (index: Int, T) -> Iterable<R>)") {
-        includeDefault()
-        include(Maps, CharSequences, ArraysOfUnsigned)
-    } builder {
-        inlineOnly()
+    val f_flatMapIndexed = listOf(Iterables, Sequences).map { containerFamily ->
+        val containerClass = containerFamily.name.dropLast(1)
+        fn("flatMapIndexed(transform: (index: Int, T) -> $containerClass<R>)") {
+            when (containerFamily) {
+                Iterables -> include(Iterables, Sequences, ArraysOfObjects, ArraysOfPrimitives, ArraysOfUnsigned, CharSequences)
+                Sequences -> include(Sequences, Iterables, ArraysOfObjects)
+            }
+        } builder {
+            inlineOnly()
 
-        doc {
-            """
-            Returns a single ${f.mapResult} of all elements yielded from results of [transform] function being invoked on each ${f.element}  
-            and its index in the original ${f.collection}.
-            """
-        }
-        sample("samples.collections.Collections.Transformations.flatMap")
-        typeParam("R")
-        returns("List<R>")
-        body {
-            "return flatMapIndexedTo(ArrayList<R>(), transform)"
-        }
-        specialFor(Sequences) {
-            inline(Inline.No)
-            signature("flatMapIndexed(transform: (index: Int, T) -> Sequence<R>)")
-            returns("Sequence<R>")
+            since("1.4")
+            doc {
+                """
+                Returns a single ${f.mapResult} of all elements yielded from results of [transform] function being invoked on each ${f.element}  
+                and its index in the original ${f.collection}.
+                """
+            }
+            annotation("@OptIn(kotlin.experimental.ExperimentalTypeInference::class)")
+            annotation("@OverloadResolutionByLambdaReturnType")
+            if (family != ArraysOfUnsigned)
+                annotation("""@kotlin.jvm.JvmName("flatMapIndexed$containerClass")""")
+            sample("samples.collections.Collections.Transformations.flatMapIndexed")
+            typeParam("R")
+            returns("List<R>")
             body {
-                """
-                return sequence {
-                    var index = 0
-                    for (element in this@flatMapIndexed) {
-                        val result = transform(checkIndexOverflow(index++), element)
-                        for (r in result) yield(r)
-                    }
+                "return flatMapIndexedTo(ArrayList<R>(), transform)"
+            }
+            specialFor(Sequences) {
+                inline(Inline.No)
+                returns("Sequence<R>")
+                body {
+                    "return flatMapIndexed(this, transform, $containerClass<R>::iterator)"
                 }
-                """
             }
         }
     }
 
-    val f_flatMapIndexedTo = fn("flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Iterable<R>)") {
-        includeDefault()
-        include(Maps, CharSequences, ArraysOfUnsigned)
-    } builder {
-        inlineOnly()
 
-        doc {
-            """
-            Appends all elements yielded from results of [transform] function being invoked on each ${f.element} 
-            and its index in the original ${f.collection}, to the given [destination].
-            """
-        }
-        specialFor(Sequences) {
-            signature("flatMapIndexedTo(destination: C, transform: (index: Int, T) -> Sequence<R>)")
-        }
-        typeParam("R")
-        typeParam("C : MutableCollection<in R>")
-        returns("C")
-        body {
-            fun checkOverflow(value: String) = if (f == Sequences || f == Iterables) "checkIndexOverflow($value)" else value
-            """
-            var index = 0
-            for (element in this) {
-                val list = transform(${checkOverflow("index++")}, element)
-                destination.addAll(list)
+    val f_flatMapIndexedTo = listOf(Iterables, Sequences).map { containerFamily ->
+        val containerClass = containerFamily.name.dropLast(1)
+        fn("flatMapIndexedTo(destination: C, transform: (index: Int, T) -> $containerClass<R>)") {
+            when (containerFamily) {
+                Iterables -> include(Iterables, Sequences, ArraysOfObjects, ArraysOfPrimitives, ArraysOfUnsigned, CharSequences)
+                Sequences -> include(Sequences, Iterables, ArraysOfObjects)
+            }
+        } builder {
+            inlineOnly()
+
+            since("1.4")
+            doc {
+                """
+                Appends all elements yielded from results of [transform] function being invoked on each ${f.element} 
+                and its index in the original ${f.collection}, to the given [destination].
+                """
+            }
+            annotation("@OptIn(kotlin.experimental.ExperimentalTypeInference::class)")
+            annotation("@OverloadResolutionByLambdaReturnType")
+            if (family != ArraysOfUnsigned)
+                annotation("""@kotlin.jvm.JvmName("flatMapIndexed${containerClass}To")""")
+            typeParam("R")
+            typeParam("C : MutableCollection<in R>")
+            returns("C")
+            body {
+                fun checkOverflow(value: String) = if (f == Sequences || f == Iterables) "checkIndexOverflow($value)" else value
+                """
+                var index = 0
+                for (element in this) {
+                    val list = transform(${checkOverflow("index++")}, element)
+                    destination.addAll(list)
+                }
+                return destination
+                """
             }
-            return destination
-            """
         }
     }